Hi
Warning: this goes on quite a bit. It contains early-morning
caffeinated ramblings and many "hmmm I wonder what this does..."
snippets.
I'm looking for the best way to parameterise shared examples. Imagine
(as an academic example...) you were doing it for subclasses of Struct
instances (a more realistic example might be ActiveRecord subclasses, or
DataMapper resources), such as:
class MyStruct < Struct.new(:a, :b)
end
class MyOtherStruct < Struct.new(:foo, :bar)
end
I've seen it done with #let, eg:
shared_examples_for "a Struct" do
it "has methods" do
properties.each do |property|
struct.should respond_to(property)
end
end
end
describe MyStruct do
let(:struct) { MyStruct.new }
let(:properties) { [:a, :b] }
it_should_behave_like "a Struct"
end
describe MyOtherStruct do
let(:struct) { MyOtherStruct.new }
let(:properties) { [:foo, :bar] }
it_should_behave_like "a Struct"
end
Which is not a bad solution, but does feel a bit too much like using
(scoped) global variables for my liking. There's no explicit
association between the shared examples and their parameters (and the
arguments actually passed in each example group.
So I started to wonder if this could be done with metadata. My first
naive stab was this:
describe MyStruct do
it_should_behave_like "a Struct", properties: [:a, :b]
end
But this fails:
Could not find shared example group named {:properties=>[:a, :b]}
Anyway, I dug in a bit and found that the metadata is only available to
the example group anyway, not the examples themselves. So you can't do:
describe MyStruct, properties: [:a, :b] do
let(:struct) { MyStruct.new }
it "has methods" do
metadata[:properties].each do |property|
struct.should respond_to(property)
end
end
end
But (more digging), you can do this:
describe MyStruct, properties: [:a, :b] do
let(:struct) { MyStruct.new }
it "has methods" do
example.metadata[:properties].each do |property|
struct.should respond_to(property)
end
end
end
Which means I can get this close to my original dreamed-up syntax:
shared_examples_for "a Struct with metadata" do
it "has methods" do
example.metadata[:properties].each do |property|
struct.should respond_to(property)
end
end
end
describe MyStruct, properties: [:a, :b] do
let(:struct) { MyStruct.new }
it_should_behave_like "a Struct with metadata"
end
I don't object so much to having "struct" floating around, as it's
fairly safe to say all the shared examples will depend on #struct being
available. Although, arguably, #subject would be better:
shared_examples_for "a subject Struct with metadata" do
it "has methods" do
example.metadata[:properties].each do |property|
subject.should respond_to(property)
end
end
end
describe MyStruct, properties: [:a, :b] do
subject { MyStruct.new }
it_should_behave_like "a subject Struct with metadata"
end
or even:
shared_examples_for "a subject Struct with metadata" do
metadata[:properties].each do |property|
it { should respond_to(property) }
end
end
describe MyStruct, properties: [:a, :b] do
subject { MyStruct.new }
it_should_behave_like "a subject Struct with metadata"
end
I tried to be a bit clever to see if I could clean up the example
definitions in the shared spec, but I got this far before hitting
weirdness that was beyond my understanding of RSpec (and the reach of my
spade...). But, this was a bit of a tangent anyway:
shared_examples_for "a subject Struct with metadata" do
metadata[:params].each { |key, value| define_method(key) { value } }
p self.inspect # outputs nil (!!!)
properties.each do |property|
it { should respond_to(property) }
end
end
describe MyStruct, params: {properties: [:a, :b]} do
subject { MyStruct.new }
it_should_behave_like "a subject Struct with metadata"
end
Sooooo... after all this, I just wondered if anyone had any ideas what
the best way to achieve this is, and how it could be extended.
For example, would there be any merit in being able to write:
it_should_behave_like "a Struct", properties: [:a, :b]
?
Also I figure that as the metadata system is new, it's potentially
unfinished and/or in flux. What are the plans/intentions/opportunities
for expansion for it?
Cheers
Ash
--
http://www.patchspace.co.uk/http://www.linkedin.com/in/ashleymoran

El 23/07/2010, a las 09:09, Ashley Moran
escribió:
>> let(:properties) { [:a, :b] }> it_should_behave_like "a Struct"> end>> describe MyOtherStruct do> let(:struct) { MyOtherStruct.new }> let(:properties) { [:foo, :bar] }> it_should_behave_like "a Struct"> end>> Which is not a bad solution, but does feel a bit too much like using (scoped) global
variables for my liking. There's no explicit association between the shared examples and
their parameters (and the arguments actually passed in each example group.
Recently commited (RSpec 2.0.0.beta.18) was the ability to pass a block
to "it_should_behave_like", making the relation clearer; eg:
describe MyStruct do
it_should_behave_like 'a Struct' do
let(:struct) { MyStruct.new }
end
end
I did ask about parametrizing that explicitly via metadata, but David
feels that the block based approach is better; see the full thread here:
http://github.com/rspec/rspec-core/issues/71
Wincent